My wife's purse had an outside zipper area that would open up to a padded, generic holster velcroed in place inside the pocket. It was sold out of a gun shop. I would think if you are making a custom bag for someone, you could make a removable holster attached with velcro to hold it in place that was specific to the firearm. Then, should they ever change the firearm they carry to another model, they come back to you for another insert.